In order to make a fleece of extrudable synthetic-resin filaments it is standard to plastify the resin and extrude it through a row of tiny holes in a die as individual filaments. The row of holes is flanked by two slot nozzles that emit jets of pressurized air that meet at an acute angle on a line normally directly below the holes. Thus the filaments that exit will be buffeted by these air blasts and will therefore be broken up and deposited below the apparatus on a substrate, normally a moving belt, in a random mass that cures to form a nonwoven mat or fleece.
The way the air jets interact with the emerging monofilaments determines many of the qualities of the fleece. If the filaments are broken up into small pieces, the fleece will be fairly dense, and if they are not broken up at all but instead the air blasts merely cure and harden them so they deposit as endless monofilaments, a looser and spongier fleece will be the result.
In European patent application 377,926 of P. G. Buehning a system of the above-identified type is shown where the compressed air is fed via inlet passages to chambers that open directly at the slit nozzles. The chambers have a larger flow cross section than the sum of the flow cross sections of the nozzles but are not set up as diffusers. Even though some form of flow diverter is provided in the inlet passage upstream of each chamber, there is some variation in pressure along the length of the slit nozzles.
The pressure must not vary even a very small amount along the length of the slit nozzle to produce a uniform product. Indeed, the major factor affecting the product is in fact the air blast on the emerging filaments so that any slight variation will produce a different type of filament and, hence, a strip of different consistency along the nonwoven product that is invariably drawn away in a direction perpendicular to the row of filament-emitting nozzles.